1. Field of the Invention
The present invention relates to unsaturated elastomeric AB block copolymers based on butadiene, isoprene and styrene, their manufacture and their use for the manufacture of tire treads.
2. Discussion of the Background
It is generally accepted that rubber polymers which are to be employed in tire making must satisfy the following requirements, i.e., cold creep must be as little as possible, the polymers must be readily processable in subsequent blending processes, the polymers must be flowable during the molding processes, and the polymers must be readilly vulcanizable.
In addition, special requirements have to be complied with which arise from their particular application in tire making. It is well known that in recent times increased demands are being made on the properties of tire treads:
(a) They are required to retain their elasticity even at low temperatures. PA1 (b) They must exhibit good anti-skid properties in wet conditions. PA1 (c) They are required to have high abrasion resistance to provide a correspondingly long service life. PA1 (d) When subjected to dynamic loads they should generate as little heat as possible. Their rolling resistance is to be as low as possible in order to keep the fuel consumption of the vehicle as low as possible. PA1 1. The block copolymers do not satisfy adequately the abovementioned requirements with a view to their use as tire materials. PA1 2. Compatibility problems of the two blocks are experienced. PA1 3. The tan delta curve exhibits only a narrow damping maximum. PA1 4. Large amounts of comparatively expensive isoprene are required. PA1 (i) 40 to 80 wt.% of a block A containing butadiene units with a content of uniformly distributed vinyl groups of 8 to 60%, and PA1 (ii) 60 to 20 wt.% of a block B which contains 0-60 wt.% 1,3-butadiene, at least 10 wt.% isoprene and up to 45 wt.% styrene, the diene units (1,3-butadiene and isoprene) having a vinyl content of 75 to 90%.
It is known that rubbers, when subjected to torsional vibration tests exhibit a temperature dependency of the logarithmic decrement of mechanical damping and derived therefrom a temperature dependency of the mechanical loss factor tan delta which when expressed as a graph, yields a graph configuration which is characteristic for the particular rubber. The desired requirements for tire treads are met in particular if the tan delta curve comprises as wide a rane of high vibration as possible (see K. H. Nordsiek. Kautschuk und Gummi, Kunstoffe 38, 178 (1985) and 39, 599 (1986).
It is also known that these partly contradictory properties of tire threads are determined to a substantial extent by the nature and composition of the rubbers employed for this purpose. Homopolymers based on the conventionally empolyed monomeric raw materials such as butadiene, isoprene and styrene do not meet these requirements satisfactorily (see EP-OS 0 054 204 and JP-OS 82/87 406).
Blends of different types of rubbers are in practice subJect to the disadvantage that the above stated spectrum of properties is not attained and the desired tire technological qualities are not reproduced reliably. Accordingly, there exists a need for rubbers which substantially satisfy the aforesaid desired properties. In principle it should be possible to attain this object with rubbers composed of polymers comprising a variety of blocks.
For purposes of this invention, the meaning of blocks of a polymer is not restricted only to chain segments composed of different monomeric building elements, but also includes those segments which, dictated by the extraneous process parameters, exhibit abrupt variations in the nature of incorporation of the monomeric building elements or in the proportion in which they are incorporated in a chain segment.
Although the butadiene-isoprene copolymer described in EP-0S 0 054 204 comprises in its initial and terminal portion a different content of isoprene as a result of the lower tendency of isoprene to polymerize as compared with butadiene, it is not to be considered a block copolymer within the meaning just explained.
Also, if during the copolymerization of dienes and styrene the styrene proportion is changed (see DE-OS 31 08 583) no block copolymers are attained, but merely a gradual transition. The desired improvement of tire technological properties is still inadequate, even 1n that case. Single phase rubber systems are described in DE-OS 31 08 583 comprising a damping maximum created by a glass transition point in a very narrow temperature range.
An improvement is attained only by virtue of a copolymer being produced comprising two different blocks A and B which differ in their structure and/or composition.
A statistical styrene-butadiene block copolymer is thus described for example in DE-OS 31 51 139. The blocks differ in their butadiene contents and their contents of vinyl bonds. They are so intermixed that they are rendered compatible and that instead of two separate damping maxima only a single such maximum is observed.
In DE-OS 35 30 438 rubber compositions are claimed which comprise at least 20% of a styrenebutadiene block copolymer. The blocks differ in respect of their styrene contents, their vinyl bond contents and, as a result thereof, in their glass transition temperatures. In that case as well, the tan delta curve exhibits only a narrow temperature range of maximum damping.
Japanese published specification 83/122 907 describes branched rubber polymers which may be obtained by the conversion e.g. of a metallic tetrahalogen compound, such as SnCl.sub.4 with block copolymers comprising a polyisoprene and a polybutadiene block. Thus each of the two blocks is present as a homopolymer. The starshaped polymer which is attained after conversion with the metallic coupling agent forms a single phase system having one narrow glass transition point.
GB-PS 2 090 840 describes block copolymers which are attained by the polymerization of dienes or the copolymerization of diene mixtures and the blocks of which differ in respect of their contents of 1,2 and/or 3,4 structural units by 20 to 50 mol %. The preparation of such block copolymers takes place in the presence of different amounts of co-catalyst or at different temperatures.
Tire treads are described in EP-OS 0 173 791, the rubber component of which may be composed to 30 to 100% of block copolymers based on butadiene, isoprene and optionally styrene and/or piperylene. The block copolymers are produced in the presence of cocatalysts by increasing the temperature and may, for example, comprise an AB structure. The polymers always contain a terminal butadiene block which is formed at rising temperature and which accordingly comprises a comparatively high content of 1,2 structural units and an uneven distribution of the vinyl groups. Even those block copolymers do not yield tan delta curves having an adequately wide plateau in order to comply optimally with all required tire properties (see comparative example A). Accordingly, even that specification proposes to blend the block copolymers so obtained with other rubber components (see Claim 1 and Example 2).
All aforementioned block copolymers are subject to at least one of the following shortcomings: